1. Field of the Invention
This invention relates to an image forming apparatus using the electrophotographic process, and particularly to an image forming apparatus such as a copier, a printer or a facsimile apparatus.
2. Related Background Art
An image forming apparatus adopting the electrophotographic process has a plurality of image forming portions for forming latent images on photosensitive drums which are image bearing members by the utilization of light, magnetism, charges or the like, and visualizing the latent images to thereby obtain visible images, transfer material conveying means for conveying a transfer material to a transfer position of each image forming portion, and fixing means for fixing the image transferred onto the transfer material on the transfer material.
As the transfer material conveying means, use is often made of a belt-shaped conveying member, i.e., a transfer material conveying belt (hereinafter simply referred to as the "conveying belt") from the viewpoint of the conveying property, and it conveys the transfer material to the transfer position, and further to the fixing device. At this time, the transfer material is retained on the conveying belt by an electrostatic attractive force and is conveyed.
The conveying belt is passed over, supported and moved by a drive roller (drive rotary member) and at least one driven roller (driven rotary member).
Contrivance is made such that the conveying belt is extended with a constant force between the drive roller and the driven roller, and the conveying belt is moved at a constant speed from the most upstream image forming portion to the most downstream image forming portion without slackening between the two rollers.
The transfer material is electrostatically attracted to the conveying belt as previously described, and is conveyed to each image forming portion while being retained on the conveying belt, and a toner image of each color is transferred to the transfer material, and is fixed at fixing means, whereby a full color image can be obtained.
Also, the conveying belt is provided with deviation regulating means for blocking the deviation of the belt (the movement of the belt on the roller shaft in a direction orthogonal to the driving direction) by the extraneous force during long-time rotation or jam clearance to thereby prevent the belt from falling off the rollers.
The image forming process in an image forming apparatus according to the conventional art will now be described specifically with reference to FIG. 23 of the accompanying drawings. FIG. 23 schematically shows an example of a full color image forming apparatus according to the conventional art.
First, second, third and fourth image forming portions Pa, Pb, Pc and Pd are juxtaposed in the main body of the image forming apparatus. Below the first to fourth image forming portions Pa to Pd from cassettes 107a and 107b containing transfer materials therein to a fixing device 111 in the main body of the image forming apparatus, there is provided an endless belt-shaped transfer material conveying means, i.e., a conveying belt 108, circulatorily moved to convey the transfer material through these image forming portions Pa-Pd. This conveying belt 108 is passed over a driven roller 109 and a drive roller 110. Further, the driven roller 109 is provided with tension imparting means 109t for imparting tension to the conveying belt 108, and deviation regulating means, not shown.
The conveying belt 108 is a thin belt of a dielectric material driven in the direction indicated by the arrow A by the drive roller 110 rotated by a drive motor, e.g. a stepping motor 120, and bears thereon the transfer material fed out of the cassette 107a or 107b and fed in synchronism with image formation by a pair of registration rollers 100, and the transfer material is conveyed to the afore described image forming portions Pa-Pd in succession. The fixing device 111 receives the transfer material discharged from the drive roller 110, and collectively fixes toner images of respective colors transferred to the transfer material in superposed relationship with one another at the image forming portions Pa-Pd to thereby form a permanent image.
Each portion of the image forming apparatus will now be described in detail. The image forming portions Pa-Pd have substantially the same construction, and include photosensitive drums 101a, 101b, 101c and 101d which are image bearing members rotatively driven in the direction indicated by arrow B, and primary chargers 102a, 102b, 102c and 102d for uniformly is charging the photosensitive drums 101a-101d, respectively, developing devices 103a, 103b, 103c and 103d for developing electrostatic latent images formed on the photosensitive drums 101a-101d, respectively, transfer chargers 104a, 104b, 104c and 104d for transferring the developed toner images of respective colors to the transfer material, and cleaning devices 105a, 105b, 105c and 105d for removing the toners remaining on the photosensitive drums 101a-101d, respectively, are successively disposed around the respective photosensitive drums 101a-101d in the direction of rotation thereof (the direction indicated by the arrow B). Exposing optical systems 106a, 106b, 106c and 106d are provided above the photosensitive drums 101a-101d, respectively.
Yellow (Y) toner, magenta (M) toner, cyan (C) toner and black (K) toner are contained in the developing devices 103a, 103b, 103c and 103d, respectively.
In the above-described full color image forming apparatus, when a transfer material is placed on the conveying belt 108, the image forming processes for the corresponding photosensitive drums 101a-101d are successively started with the movement of the conveying belt 108 in the direction indicated by the arrow A. That is, a yellow image is formed on the photosensitive drum 101a of the first image forming portion Pa, a magenta image is formed on the photosensitive drum 101b of the second image forming portion Pb, a cyan image is formed on the photosensitive drum 101c of the third image forming portion Pc, and a black image is formed on the photosensitive drum 101d of the fourth image forming portion Pd.
The transfer material passes below the photosensitive drums 101a-101d in the first to fourth image forming portions Pa-Pd and is conveyed toward the fixing device 111 by the movement of the conveying belt 108, and the toner images of the respective colors are transferred to the transfer material in superposed relationship with one another by the transfer chargers 104a-104d of the image forming portions Pa-Pd, whereby a full color image is formed.
The transfer material passes through the last fourth image forming portion Pd, whereafter it is separated from the conveying belt 108 and is sent to the fixing device 111, in which the toner images of the respective colors transferred in superposed relationship with one another are fused and fixed, whereafter the transfer material is discharged to a discharge tray 112.
However, with the ideal conveying speed of the conveying belt 108 as the premise, latent images are formed on the corresponding photosensitive drums 101a-101d at exposing time intervals corresponding to desired image information, whereby a desired transferred image ought to be formed on the transfer material, but since there are the following problems, positional deviation may occur to the transferred image to deteriorate the quality of the image.
The following two points become great problems in driving the conveying belt 108 so as to assume an ideal conveying speed.
(1) The conveying speed of the transfer material conveyed by the conveying belt 108 is sequentially changed by the geometrical shape difference or the like of the drive roller 110 or between the drive roller 110 and the driven roller 109 and therefore, in the image sequentially formed on the transfer material, a fluctuation occurs to the image in the direction of movement of the conveying belt and the quality of the image formed on the transfer material is liable to be deteriorated.
The speed V of the surface of the conveying belt prescribed by the drive roller 110 having a radius r driven at a predetermined angular speed w and the thickness h of the conveying belt 108 is represented as follows: EQU V=(r+h/2).multidot.w (1)
When eccentricity .DELTA.r is superposed on this drive roller 110, the speed fluctuation .DELTA.V of the conveying speed V prescribed by the drive roller 110 is represented as follows: EQU .DELTA.V.sub.w =.DELTA.r.sub.w.multidot.w (2)
By the speed fluctuation .DELTA.V by the eccentricity of this drive roller, the transferred image causes misregister, i.e., misregister of colors.
However, such misregister of the period of the drive roller creates misregister having the similar periodicity of the respective colors by making, for example, the diameter of the drive roller equal to the photosensitive drum pitch, but it is possible to suppress the creation of the misregister among the respective colors.
(2) Also by the fluctuation of the conveying belt 108 in the direction of thickness thereof over the full circumferential length thereof, a change occurs to the conveying speed of the transfer material prescribed by the drive roller 110, and this has led to the problem that with the deterioration of the quality of image in which the image on a single transfer material deviates from an ideal position, a fluctuation also occurs to the images on a plurality of transfer materials and the repetitive positional reproducibility among the transfer materials is deteriorated.
Assuming that for example, a thickness fluctuation .DELTA.h exists over the full circumferential length in the thickness h of the conveying belt wound on the drive roller 110 having a radius r and driven at a constant angular speed w, the speed fluctuation .DELTA.V in the speed V of the conveying belt prescribed by the drive roller 110 is represented as follows: EQU .DELTA.V.sub.L =.DELTA.h.sub.L.multidot.w (3)
L: the period of the full circumferential length of the belt
The relation between the fluctuation in the linear speed of the conveying belt prescribed by the drive roller 110 of the conveying belt and the positional deviation of the image formed thereby can be typically represented as shown in FIG. 24 of the accompanying drawings.
The axis of abscissas indicates time t, the axis of ordinates indicates the fluctuation v in the linear speed of the conveying belt, small letters y1-k1 indicate exposure timing, and the manner in which capital letters Y1-K1 are transferred to the first transfer material T1 is typically shown. At this time, the position of the exposed image on the transfer material is indicated by continuous cross lines like +++++ so as to be capable of being distinctly discriminated.
This is singly effected, or is repetitively carried out up to a suffix n. Also, at this time, the full circumferential length of the conveying belt 108 corresponds to L.
A state in which the positional deviation (misregister of colors) of the toner images of respective colors attributable to the fluctuation of the speed based on the fluctuation of the thickness of the conveying belt 108 in the circumferential direction thereof occurs in T1, T2, . . . , Tn and the positional deviation also occurs between T1-T2 and between T2-Tn is shown as the position of arrows Y1-K1, and typically shows that the interval between the cross lines is an unequal interval.
At this time, a fluctuation indicated by solid line L1 which is a fine AC component in the fluctuation in the speed of the conveying belt corresponds to item (1) above, and a large undulating component indicated by broken line corresponds to item (2) above.